Scheduling acquisition attempts of service providing systems

ABSTRACT

Methods, apparatuses and computer program products for causing a subscriber station ( 100 ) to attempt to acquire one of a plurality of systems. The subscriber station ( 100 ) first detects an event that triggers use of a system acquisition schedule. The system acquisition schedule, which can be time based or sequence based, defines when to attempt to acquire a desired system. The subscriber station ( 100 ) attempts to acquire the desired system in accordance with the system acquisition schedule. The subscriber station ( 100 ) performs other functions when the system acquisition schedule does not indicate an attempt should be made to acquire the desired system. Such other functions can include attempting to acquire other systems when the system acquisition schedule does not indicate an attempt should be made to acquire the desired system.

BACKGROUND OF THE INVENTION

[0001] I. Field of the Invention

[0002] The present invention relates to wireless communication systems.More particularly, the present invention relates to a novel and improvedmethod and apparatus for scheduling acquisition attempts of one or moreservice providing systems in a subscriber station capable of operationwith a plurality of communication systems.

[0003] II. Related Art

[0004] As mobile communication systems become more prevalent in society,the demands for greater and more sophisticated service have grown. Tomeet the capacity needs of mobile communication systems, techniques ofmultiple access to a limited communication resource have been developed.The use of code division multiple access (CDMA) modulation techniques isone of several techniques for facilitating communications in which alarge number of system users are present. Other multiple accesscommunication system techniques, such as time division multiple access(TDMA) and frequency division multiple access (FDMA) are known in theart. However, the spread spectrum modulation technique of CDMA hassignificant advantages over these modulation techniques for multipleaccess communication systems.

[0005] The use of CDMA techniques in a multiple access communicationsystem is known in the art and is disclosed in U.S. Pat. No. 4,901,307,issued Feb. 13, 1990, entitled “SPREAD SPECTRUM MULTIPLE ACCESSCOMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, assignedto the assignee of the present invention. The use of CDMA techniques ina multiple access communication system is further disclosed in U.S. Pat.No. 5,103,459, issued Apr. 7, 1992, entitled “SYSTEM AND METHOD FORGENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, alsoassigned to the assignee of the present invention.

[0006] In order for service to be provided using a subscriber station,the subscriber station must first acquire a service providing system.Such service providing systems can be identified, for example, by mode(for example, CDMA, TDMA, GSM, etc.) and frequency. Many wirelesssubscriber stations (also known as wireless communications devices, userterminals, or mobile stations) include tables and/or lists that are usedby a subscriber station when it is attempting to obtain service. Forexample, a subscriber station can maintain a preferred roaming list(PRL) that is maintained in such a manner that the subscriber stationcan readily determine which systems (preferred or negative) cover commongeographical regions (that is, areas of common radio coverage).Moreover, the PRL can rank systems from most desirable to leastdesirable. Accordingly, subscriber stations can prioritize theirattempts to acquire service such that they attempt to acquire service onmore desirable systems before attempting to acquire service on lessdesirable systems.

[0007] Subscriber stations can also include tables or lists of the N(for example, N=10) most recently used systems (that is, the systemsthat were most recently used to provide service). Such a table or listis typically referred to as a most recently used (MRU) table. Asubscriber station can, for example, attempt initial system acquisition(for example, after a power up) using the MRU table.

[0008] Subscriber stations typically use one or more of the abovediscussed tables (and/or other similar types of tables) to determine theorder in which to attempt to acquire systems. For example, after powerup, a subscriber station may attempt to acquire a system based on anorder specified by an MRU table and/or a PRL. Similarly, after a call isdropped (for example, due to loss of signal or signal fading), asubscriber station may attempt to acquire (including re-acquire) asystem based on an order specified by the MRU table and/or the PRL. Morespecifically, upon detecting specific events, such as power up or systemloss, a subscriber station may be programmed such that the first systemthe subscriber station attempts to acquire (specifically in this case,re-acquire) is the last system upon which service was successfullyprovided (that is, the most recently used system). An example of anotherevent that can be detected is the loss of a paging channel (for example,due to loss of signal or signal fading) while the subscriber station isin idle mode.

[0009] For example, after a subscriber station is powered up, thesubscriber station first attempts to acquire service using the mode andfrequency of the system upon which service was last provided when thesubscriber station was last powered on (that is, prior to power down).In other words, the subscriber station first attempts to re-acquire themost recently used system before attempting to acquire one or more othersystems. In this example, the acquisition of the most recently usedsystem is attempted because it is likely that the subscriber station hasremained within the same geographic region, and thus, that service canagain be provided using the most recently used system.

[0010] Similarly, after a subscriber station drops a call due to loss ofsignal or signal fading, the subscriber station immediately attempts tore-acquire service using the mode and frequency of the system upon whichservice was being provided when the call was dropped. In this example,the re-acquisition of the most recently used system is attempted becauseit is likely that the cause of the system loss is only temporary, andthus, that service can again be provided using the most recently usedsystem. For example, a subscriber station may lose system acquisitionbecause the user of the subscriber station travels on an elevator for afew seconds, travels behind a service blocking building for a fewseconds, or travels in an underground tunnel for a few minutes. In eachof these examples, successful re-acquisition of the most recently usedsystem is likely once the user, for example, exits the elevator, is nolonger behind the building, or exits the underground tunnel.

[0011] Some prior subscriber stations, designed by the assignee of thepresent invention, are adapted to alternate between the most recentlyused system and other systems when attempting to acquire (includingre-acquire) a system. More specifically, the subscriber station's firstattempt is to acquire service on the most recently used system, thesecond attempt is to acquire an alternative system, the third attempt toacquire the most recently used system, the fourth attempt is to acquireanother alternative system, the fifth attempt to acquire the mostrecently used system, the sixth attempt is to acquire a furtheralternative system, and so on. The order in which the subscriber stationattempts to acquire service on the alternative systems can be based onvarious algorithms that are known in the art. Once all alternativesystems are attempted in accordance with one of such algorithms, theabove described alternating with the most recently used system isstopped and acquisition is attempted based only on an algorithm thatdoes not give such repeated high priority to the most recently usedsystem. Even though the above described method of alternating betweenthe most recently used system and alternative systems (referred tohereafter as the “alternating method”) has proved useful, thisalternating method has some drawbacks.

[0012] First, since the alternating method is based on an alternatingsequence scheme, it is dependent on the processing speed of thesubscriber station's hardware. More specifically, if a subscriberstation operates at a high speed, the subscriber station may alternatebetween the most recently used system and all of the hypothesizedavailable alternative systems in only a few seconds. As mentioned above,the alternating method stops after unsuccessful attempts to acquire allthe hypothesized available alternative systems. Thus, the subscriberstation may stop giving the most recently used system repeated highpriority earlier than it should. For example, while the subscriberstation is located in an elevator, the subscriber station may completethe alternating method and stop giving the most recently used systemrepeated high priority even though the most recently used system wouldbe immediately available once the subscriber station exits the elevator.In summary, the subscriber station forgets about the lost system tooquickly.

[0013] Second, since the alternating method is based on an alternatingsequence scheme, it may give too much priority to the most recently usedsystem, thereby wasting acquisition attempts and slowing down the amountof time necessary to acquire an alternative system. For example, assumea user of a subscriber station walks behind a large building that blocksservice (or enters an elevator or tunnel) thereby causing the subscriberstation to drop a call. Using the alternating method, the subscriberstation may, for example, attempt to acquire the most recently usedsystem three separate times, and alternative systems three times, withinless than a second (depending on the processing speed of the subscriberstation). However, it is unlikely that the subscriber station will beable to acquire the most recently used system if, for example, 0.3seconds earlier the subscriber station was unable to acquire thatsystem. In summary, the subscriber station spends too much timeattempting to re-acquire the lost system, while starving other systems.

[0014] Accordingly, there is a need to improve the way subscriberstations attempt to acquire (including, re-acquire) systems.

SUMMARY OF THE INVENTION

[0015] The present invention is directed toward methods, apparatuses,and computer program products for improving the way a subscriber stationattempts to acquire a service providing system. The present inventiongives appropriate priority to specific systems (such as the mostrecently used system) without giving undue or too much priority to suchspecific systems.

[0016] A method of the present invention includes the step of detectingan event that triggers use of a system acquisition schedule. The systemacquisition schedule defines when to attempt to acquire a desiredsystem. The subscriber station attempts to acquire the desired system inaccordance with the system acquisition schedule. The subscriber stationperforms other functions when the system acquisition schedule does notindicate to attempt to acquire the desired system. The present inventioncan also include the step of selecting the system acquisition schedulefrom a plurality of system acquisition schedules, wherein the selectingis based on the triggering event.

[0017] According to the present invention, a subscriber station canattempt to acquire more than one desired system in accordance with morethan one system acquisition schedule. For example, while attempting toacquire a first desired system in accordance with a first systemacquisition schedule, the subscriber station can detect another eventthat triggers use of a second system acquisition schedule. The secondsystem acquisition schedule defines when an attempt should be made toacquire a second desired system. The subscriber station can then alsoattempt to acquire the second desired system in accordance with thesecond system acquisition schedule.

[0018] The subscriber station may attempt to acquire other systems whenthe system acquisition schedule does not indicate an attempt should bemade to acquire the desired system. When attempting to acquire othersystems the subscriber station follows alternative algorithms forprioritizing systems. It is possible that one of the “other” systems canbe the desired system (if, for example, a PRL specifies an attemptshould be made to acquire that system). However, in a specificembodiment, each of the other systems is different from the desiredsystem. For example, if each system is defined by a mode/frequencycombination, and the desired system is defined by a specifiedmode/frequency combination, then each of the other systems is defined bydifferent mode/frequency combinations than the desired system.

[0019] System loss can be the event that triggers use of the systemacquisition schedule. In this situation, the system acquisition schedulecan define when an attempt should be made to reacquire the desiredsystem whose loss triggers use of the system acquisition schedule. Apower up of a subscriber station is an example of another triggeringevent. In this situation, the system acquisition schedule can definewhen an attempt should be made to acquire the desired system, thedesired system being the system that was most recently used by thesubscriber station.

[0020] A system acquisition schedule can be time or sequence based. Morespecifically, a system acquisition schedule can define specified timesat which the subscriber station should attempt to acquire the desiredsystem. Alternatively, a system acquisition schedule can define specificsequence counts according to which the subscriber station should attemptto acquire the desired system. The subscriber station performs otherfunction at times or counts other than the specified times or countsdefined by the system acquisition schedule. The “other functions” canbe, as mentioned above, attempting to acquire other systems.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

[0021] The features, objects, and advantages of the present inventionwill become more apparent from the detailed description set forth belowwhen taken in conjunction with the drawings in which like referencecharacters identify the same or similar elements throughout and wherein:

[0022]FIG. 1 is a block diagram of an exemplary multi-mode subscriberstation for which the present invention is useful;

[0023]FIG. 2 is a high level flow diagram illustrating an overview ofthe embodiments of the present invention;

[0024]FIG. 3 is a flowchart depicting an exemplary embodiment of thepresent invention that utilizes time based system acquisition schedules;

[0025]FIGS. 4A and 4B illustrate timelines that are useful forexplaining the embodiment of the present invention described withreference to FIG. 3;

[0026]FIG. 5 is a flowchart depicting an exemplary embodiment of thepresent invention that utilizes sequence based acquisition schedules;and

[0027]FIG. 6 illustrates a sequence line that is useful for explainingthe embodiment of the present invention described with reference to FIG.5.

DETAILED DESCRIPTION OF THE INVENTION

[0028] The present invention is described as being for use in amulti-mode subscriber station. It should be noted that although thepresent invention is described in the context of a subscriber stationcapable of operation in analog and digital environments, the presentinvention is equally applicable to subscriber stations only capable ofoperation in one environment. Similarly, it should be noted that thedigital operation described in the context of code division multipleaccess (CDMA) operation is equally applicable to any digitalcommunication format such as WCDMA, TDMA, FDMA, GSM, etc. Environmentsthat support analog communications may, for example, use the AdvancedMobile Phone Service (AMPS) standards.

[0029] Referring to FIG. 1, when a multi-mode subscriber station (MMSS)100 is in a system determination substate, then the operations areconducted by a system determination processor 108. In the systemdetermination substate, system determination processor 108 selects thecommunication system upon which MMSS 100 is to attempt to performacquisition and provides the necessary parameters to the acquisitioncircuitry. System determination processor 108 may be implemented as amicroprocessor which functions under program control, for example, asdiscussed with reference to FIGS. 2, 3 and 5.

[0030] In an exemplary embodiment, MMSS 100 is a dual-mode subscriberstation capable of both analog transmission and reception, using analogmodulation and demodulation and processing circuitry (analogcircuitry)104, and digital transmission and reception, using digitalmodulation and demodulation and processing circuitry (digital circuitry)106. In the exemplary embodiment, digital circuitry 106 is code divisionmultiple access (CDMA) transmission and reception circuitry. However,other types of digital communication modes may be employed, such asWCDMA, TDMA, FDMA or GSM. The present invention is applicable tocellular communications systems, personal communication systems (PCS)and any other communication system which may provide service to asubscriber station capable of operation with various different systems.

[0031] The design of analog circuitry 104 is well known in the art andis, for example, described in detail in Mobile CellularTelecommunications Systems by William C. Y. Lee. The exemplaryembodiment of digital circuitry 106 is also well known in the art, andis described in detail in the aforementioned U.S. Pat. Nos. 4,901,307and 5,103,459.

[0032] A most recently used (MRU) table 109 contains a list ofcommunication systems that have been most recently used by MMSS 100. MRUtable 109 can include the N (for example, N=10) most recently usedsystems (that is, the systems that were most recently used to provideservice).

[0033] A preferred roaming list (PRL) 112 may contain system parametersfor all communication systems which MMSS 100 “knows” exist. In anexemplary embodiment, PRL 112 contains information regarding bothpreferred and negative systems. In an exemplary embodiment, the systemsstored in PRL 112 are grouped according to geographic region. Eachsystem listed within a geographic group is then sequentially orderedfrom most desirable to least desirable. PRL 112 may also associatesystem identifiers (SIDs) with mode/frequency combinations. PRL 112 mayalso associate SIDs with roaming indicators.

[0034] In an exemplary embodiment, MRU 109 and/or PRL 112 areimplemented in a nonvolatile memory which is retained even after MMSS100 is powered down.

[0035] Criteria for ranking systems may, for example, include cost ofservice, quality of service, support of unique features, etc. For eachsystem, MRU 109 and/or PRL 112 may contain the system identification(SID) of the system along with necessary acquisition parameters, such asband, frequency, mode and any other parameters useful for performingacquisition. Alternatively, MRU 109 may only include acquisitionparameters (such as frequency and mode) of the most recently usedsystems. In an exemplary embodiment, each system listed is tagged withan indication of whether the system is a system MMSS 100 is permitted touse (a preferred system) or a system which MMSS 100 is not permitted touse (a negative system). Each system is minimally identified by a mode(for example, analog AMPS, or CDMA digital) and a frequency (forexample, A side analog, or digital frequency channel 21), collectivelyreferred to hereafter as a “mode/frequency combination.”

[0036] When MMSS 100 is powered up, or loses service, MMSS 100 attemptsto acquire (or reacquire) a system as soon as possible. Within thisdescription, the term “acquire” is meant to encompasses the term“re-acquire.” That is, attempting to re-acquire a system is just onetype of attempt to acquire a system. As mentioned above, informationidentifying potentially available systems, including the mode andfrequency of the systems, is maintained in tables or lists such as MRU109, UST 111 and/or PRL 112. Subscriber stations (such as MMSS 100)typically use the above discussed tables or lists (and/or other similartypes of tables or lists) alone, or in combination, in order todetermine the order in which to attempt to acquire systems.

[0037] For example, after power up, MMSS 100 may attempt to acquire asystem based on an order specified by MRU table 109. Similarly, after acall is dropped (for example, due to loss of signal or signal fading),MMSS 100 may attempt to re-acquire a system based on an order specifiedby MRU table 109. More specifically, upon detecting specific events,such as power up or system loss, MMSS 100 may immediately attempt toacquire or re-acquire the last system on which service was successfullyprovided (that is, the most recently used system).

[0038] In another example, MMSS 100 builds a scan or search list usingPRL table 112, MRU table 109, selected user preferences (for example,Digital Only, Analog Only), and/or a system selection algorithm. Thescan or search list can identify each system by its mode/frequencycombination. Examples of entries in the scan list include: analog AMPS(mode)/A side (frequency); analog AMPS (mode)/B side (frequency);digital CDMA (mode)/channel 40 (frequency); digital CDMA (mode)/channel23 (frequency).

[0039] When MMSS 100 attempts to acquire an analog system (when inanalog mode), MMSS 100 (or more specifically, analog modulation andprocessing circuitry 104 in combination with system determinationprocessor 108) scans all the frequencies within a set of frequencies(for example, all frequencies in an A frequency set, or all frequenciesin the B frequency set) and identifies the channel in the set that hasthe strongest energy and the channel that has the second strongestenergy. MMSS 100 then tunes to the frequency channel having thestrongest energy and attempts to demodulate a received signal. Ifdemodulation of the signal over the strongest frequency channel is notsuccessful, then MMSS 100 tunes to the second strongest frequencychannel and attempts to demodulate a signal received on that frequency.If demodulation is successful, then the signal is said to be acquired.Once a signal is successfully acquired, then MMSS 100 detects the SIDassociated with the signal that was acquired. MMSS 100 can thendetermine whether to provide service on the acquired system (asidentified by the SID) based on, for example, whether the system is apreferred or negative system.

[0040] When a subscriber station attempts to acquire a digital system(when in digital mode), MMSS 100 (or more specifically, digitalmodulation and processing circuitry in combination with systemdetermination processor 108) tunes to a specific digital servicefrequency (for example, digital channel 23) and searches across codespace to try to find a pilot signal. If a pilot signal is found, thenMMSS 100 attempts to decode a synchronization channel (also referred toas a “sync channel”). If MMSS 100 successfully decodes the sync channel,then MMSS 100 determines a paging channel associated with the syncchannel. Depending on the specific implementation, MMSS 100 may attemptto decode the SID from the sync channel and/or the associated pagingchannel (and/or other overhead channels). Once a system is successfullyacquired, and MMSS 100 detects the SID associated with the system thatwas acquired, MMSS 100 can then determine whether to provide service onthe acquired system (as identified by the SID) based, for example, onwhether the system is a preferred or negative system.

[0041] MMSS 100 can also include one or more additional memory units 110that store, for example, the system acquisition schedules of the presentinvention.

[0042] As shall now be explained in detail, the present inventionassociates one or more systems with one or more system acquisitionschedules that define when MMSS 100 should attempt to acquire a specificdesired system. Each system, including the desired system, can bedefined by a mode/frequency combination.

[0043]FIG. 2 includes a high level flow diagram 200 of the operation ofembodiments of the present invention. Use of a system acquisitionschedule of the present invention is triggered when a subscriber station(for example, MMSS 100) detects an event (also referred to as a“triggering event”), as shown at a step 202.

[0044] At a step 203, the system acquisition schedule is selected from aplurality of system acquisition schedules based on the triggering event.The system acquisition schedule defines when the subscriber stationshould attempt to acquire a desired system. The desired system isdefined, for example, by a mode/frequency combination. A triggeringevent can be, for example, power up of the subscriber station, or lossof a system. If the triggering event is power up of the subscriberstation, then the desired system is the most recently used system. Ifthe triggering event is loss of a system, then the desired system is thesystem that was lost. A system may be considered lost, for example, if apaging channel or some other overhead channel is lost, or if a trafficchannel is lost (for example, due to signal fading). In one embodiment,any type of system loss causes the selection of the same systemacquisition schedule. Alternatively, there can exist a systemacquisition schedule specific to the different types of system losses.For example, a first system acquisition schedule may be selected when apaging channel is lost, and a second system acquisition schedule may beselected when a traffic channel is lost.

[0045] After an event triggers use of a system acquisition schedule, ata step 204 the subscriber station attempts to acquire the desired systemin accordance with the system acquisition schedule. The systemacquisition schedule can be a time based system acquisition schedule ora sequence based system acquisition schedule. More specifically, asdescribed with reference to FIGS. 3, 4A and 4B, the system acquisitionschedule defines times at which the subscriber station should attempt toacquire the desired system. Alternatively, as described with referenceto FIGS. 5 and 6, the system acquisition schedule defines sequencescounts at which the subscriber station should attempt to acquire thedesired system.

[0046] At a step 206, the subscriber station performs one or more otherfunctions when the acquisition schedule does not indicate the subscriberstation should attempt to acquire the desired system. The one or more“other functions” can be attempting to acquire other systems when thesystem acquisition schedule does not indicate that the subscriberstation should attempt to acquire the desired system. In one embodiment,it is possible that the subscriber station may attempt to acquire thedesired system while the subscriber station is attempting to acquire“other” systems. This can happen, for example, if a system selectionalgorithm specifies that an attempt should be made to acquire thedesired system. In an alternative embodiment, each of the “other”systems is defined by different mode/frequency combinations than thedesired system. That is, in the alternative embodiment the subscriberstation knows not to attempt to acquire the desired system when thesystem acquisition schedule does not indicate that the subscriberstation should attempt to acquire the desired system. Accordingly, inthe alternative embodiment the subscriber station attempts to acquireonly alternative systems than the desired system when the systemacquisition schedule does not indicate that the subscriber stationshould attempt to acquire the desired system.

[0047] Another example of an “other function” that the subscriberstation may perform (when the system acquisition schedule does notindicate that the subscriber station should attempt to acquire thedesired system) includes entering “power save” or “sleep” mode. Afurther example of an “other function” that the subscriber station mayperform (when the system acquisition schedule does not indicate that thesubscriber station should attempt to acquire the desired system)includes obtaining channel measurements (for example, measuring thepower of one or more pilot channels). These are just a few examples of“other functions,” and thus, are not meant to be limiting.

[0048] With reference to FIGS. 3, 4A and 4B, the subscriber stationattempts to acquire alternative systems at times not specified in thesystem acquisition schedule. With reference to FIGS. 5 and 6, thesubscriber station attempts to acquire alternative systems at sequencecounts not specified in the system acquisition schedule. The subscriberstation can go through an entire list (for example, a list specified bya PRL) of alternative systems more than once while attempting to acquirea desired system in accordance with a system acquisition schedule.

[0049] The time based system acquisition schedule will now be describedin more detail with reference to FIG. 3. Beginning at a step 302, theuse of a system acquisition schedule is triggered upon the detection ofa triggering event, some examples of which have been mentioned above.

[0050] At a next step 304, a pointer value i is initialized to equalzero, and a reference timer is initialized to zero.

[0051] At a next step 306, the reference timer is started so that thereference timer measures an amount of time since the triggering event.

[0052] At a next step 308, the pointer value i is updated or incrementedaccording to the assignment: i=i+1.

[0053] At a next step 310, there is a determination as to whether thereference timer value is greater than or equal to the i^(th) specifiedtime of a plurality of times (for example, N distinct times) defined bythe system acquisition schedule. For example, a system acquisitionschedule can specify that a subscriber station should attempt to acquirea desired system at 0 seconds, 1 second, 3 seconds, 8 seconds, and 15seconds after the triggering event (such as, power up). In this example,the 1^(st) specified time is 0 seconds, the 2^(nd) specified time is 1second, the 3^(rd) specified time is 3 seconds, the 4^(th) specifiedtime is 8 seconds, and the 5^(th) specified time is 15 seconds. Such anexemplary system acquisition schedule can be represented, for example,by the function command or script SCHED(PWR.UP, SYS,ACQ_(—)0_(—)13_(—)8_(—)15).

[0054] If, at step 310, the reference timer value is greater than orequal to the i^(th) specified time (for example, the 1^(st) specifiedtime of 0 seconds) defined by the system acquisition schedule (that is,if the answer to step 310 is YES), then flow goes to a step 312. At step312, the subscriber station attempts to acquire the desired system. Asdiscussed above, the desired system can be, for example, the last systemthat was used prior to the triggering event (for example, if thetriggering event is a power up), or the system that was providingservice when the triggering event occurred (for example, if thetriggering event was loss of a system). One of ordinary skill in the artwill appreciate that other desired systems can be selected while beingwithin the spirit and scope of the present invention.

[0055] At a next step 314, there is a determination as to whether systemacquisition was successful, and whether an affirmative decision was madeto provide service on the acquired system. More specifically, when asystem is successfully acquired, the subscriber station can detect theSID of the acquired system (for example, using an overhead channel). Inone embodiment, the decision is made to use the acquired system only ifthe acquired system is the desired system. In another embodiment, thedecision is made to use the acquired system if the acquired system isassociated with an acceptable or preferred (that is, non-negative) SID,even if the acquired system is not the “desired system”.

[0056] If the answer to step 314 is YES, then at a step 320 service isprovided using the acquired system. At this point, use of theacquisition schedule can be terminated, at a step 322.

[0057] If the answer to step 314 is NO, then flow returns to step 308where pointer value i is incremented. Flow then goes to step 310 wherethere is another determination as to whether the reference timer value(which is continually adjusted after the reference timer is started atstep 308) is greater than or equal to the i^(th) specified time (forexample, the 2^(nd) specified time of 1 second) defined by the systemacquisition schedule. If the answer to step 310 is YES, then flow goesto step 312 and the method proceeds as discussed above.

[0058] If, at step 310, the reference timer value is less than thei^(th) specified time defined by the system acquisition schedule (thatis, if the answer to step 310 is NO), then flow goes to a step 316. Atstep 316, the subscriber station attempts to acquire a system other thanthe desired system (for example, an alternative system). The subscriberstation may select an alternative system based on one or more of thetables or lists maintained by the subscriber station. The precise mannerin which the subscriber station selects an alternative system that thesubscriber station will attempt acquire is not important to the presentinvention.

[0059] At a next step 318, there is a determination as to whetheracquisition was successful, and whether an affirmative decision was madeto provide service on the acquired system. More specifically, when asystem is successfully acquired, the subscriber station can detect theSID of the acquired system (for example, using an overhead channel). Anaffirmative decision can then be made to use the acquired system, forexample, if the acquired system is associated with an acceptable orpreferred (that is, a non-negative) SID.

[0060] If the answer to step 318 is YES, then at step 320 service isprovided using the acquired system. At this point, use of theacquisition schedule can be terminated at step 322.

[0061] If the answer to step 318 is NO, then flow returns to step 310where there is another determination as to whether the reference timervalue is greater than or equal the i^(th) specified time of theplurality of times defined by the system acquisition schedule.

[0062] In the above discussed manner, the subscriber station willattempt to acquire the desired system each time the reference timervalue is greater than or equal to the i^(th) specified time defined bythe system acquisition schedule. If the desired system (or possibly,another acceptable system) is not acquired, then the pointer value i isincremented to produce an updated i^(th) pointer value. Acquisition ofthe desired system is not again attempted until the reference timervalue is again greater than or equal to the updated i^(th) specifiedtime defined by the system acquisition schedule. At other times (thatis, when the reference timer is less than the i^(th) specified time),the subscriber station attempts to acquire alternative systems.Accordingly, the subscriber station attempts to acquire the desiredsystem in accordance with the times specified by the system acquisitionschedule. The subscriber station attempts to acquire other systems whenthe system acquisition does not specify that the subscriber stationshould attempt to acquire the desired system. This shall now beexplained with reference to the timeline of FIG. 4A.

[0063] Referring to FIG. 4A, the partially shaded areas of timeline 400Arepresent times at which the subscriber station attempts to acquire thedesired system. The non-shaded areas represent times at which thesubscriber station attempts to acquire other systems. In this examplethe units of the specified times are seconds. One or ordinary skill inthe art will appreciate that units other than seconds can be used whilestill being within the spirit and scope of the present invention.

[0064] A subscriber station can be searching for more than one desiredsystem in accordance with more than one system acquisition schedule. Forexample, while the subscriber station is searching for a first desiredsystem in accordance with a first system acquisition schedule, anotherevent can trigger the use of a second system acquisition schedule. Thesecond triggering event can be, for example, a synchronization channelacquisition failure. A synchronization channel acquisition failure meansthe subscriber station detected a pilot signal, but was a unsuccessfulat decoding the synchronization channel associated with the pilotchannel. Detection of the pilot channel indicates that there is a goodprobability that there is an accessible system associated with the pilotchannel. Accordingly, a synchronization channel acquisition failure maytrigger the use of a second system acquisition schedule defining when anattempt should be made to acquire a second desired system. The seconddesired system can be specified by the mode/frequency combinationassociated with the detected pilot channel. The second systemacquisition schedule can define times at which the subscriber stationshould attempt to acquire the second desired system, wherein the timesare relative to the second triggering event. Thus, use of more than onereference timer may be implemented in parallel. This concept ofattempting to acquire systems in accordance with more than one systemacquisition schedule shall now be explained with reference to theexemplary timeline of FIG. 4B.

[0065] Referring to FIG. 4B, the partially shaded areas of timeline 400Brepresent times at which the subscriber station attempts to acquire afirst desired system in accordance with a first system acquisitionschedule triggered by a first triggering event (for example, power up).The fully shaded areas of timeline 400B represent times at which thesubscriber station attempts to acquire a second desired system inaccordance with a second system acquisition schedule triggered by asecond triggering event (for example, sync channel acquisition failure).The non-shaded areas represent times at which the subscriber stationattempts to acquire other systems.

[0066] Each of the possible triggering events that can be identified bythe subscriber station can have its own unique system acquisitionschedule. For example, the second system acquisition schedule, which istriggered by a synchronization channel acquisition failure, can have afunction command or script SCHED(SYS.ACK.FAIL, SYS, ACQ_(—)0_(—)2_(—)6).Thus, the subscriber station will attempt to acquire the second desiredsystem at 0 seconds, 2 seconds, and 6 seconds after the secondtriggering event, as shown by the fully shaded areas in example timeline400B of FIG. 4B.

[0067] In the embodiment described with reference to FIG. 3, the systemacquisition schedule defines times at which a subscriber station shouldattempt to acquire a desired system, wherein the times are all relativeto the triggering event. For example, the exemplary function command orscript SCHED(PWR.UP, SYS, ACQ_(—)0_(—)1_(—)3_(—)8_(—)15) specifies thatthe subscriber station should attempt to acquire a desired system at 0seconds, 1 second, 3 seconds, 8 seconds and 15 seconds after thetriggering event of power up. In an alternative embodiment, the systemacquisition schedule defines times between each successive attempt toacquire a desired system. For example, the exemplary function command orscript SCHED(PWR.UP, SYS, ACQ_(—)0_(—)1_(—)2_(—)5_(—)7) specifies thatthe subscriber station should attempt to acquire a desired system at 0seconds after the triggering event, then 1 second after the previousattempt (which is 1 second after the triggering event), then 2 secondsafter the previous attempt (which is 3 seconds after the triggeringevent), then 5 seconds after the previous attempt (which is 8 secondsafter the triggering event), and then 7 seconds after the previousattempt (which is 15 seconds after the triggering event). It should beappreciated that the effect of both of these examples is to attempt toacquire the desired system at equivalent times relative to thetriggering event. One of ordinary skill in the art will appreciate thatthe flow chart of FIG. 3 can be slightly modified to accommodate thisalternative embodiment. More specifically, the reference timer should beinitialized to zero after each unsuccessful attempt to acquire thedesired system (that is, after each time the answer to step 314 is NO),as shown by the dashed lines in FIG. 3.

[0068] In alternative embodiments, system acquisition schedules aresequence based, rather than time based as just described above. Morespecifically, an embodiment where the system acquisition scheduledefines sequence counts at which the subscriber station should attemptto acquire a desired system shall now be described in more detail withreference to FIG. 5. Beginning at a step 502, the use of a systemacquisition schedule is triggered upon the detection of a triggeringevent, some examples of which have been mentioned above.

[0069] At a next step 504, a pointer value i is initialized to equalzero, and a reference counter is initialized to zero.

[0070] At a next step 506, the pointer value i is updated or incrementedaccording to the assignment: i=i+1.

[0071] At a next step 508, the reference counter is updated orincremented according to the assignment: reference count=referencecount+1.

[0072] At a next step 510, there is a determination as to whether thereference count value is equal the i^(th) specified sequence count valueof a plurality of sequence count values (for example, N distinctsequence count values) defined by the system acquisition schedule. Forexample, a system acquisition schedule can specify that a subscriberstation should attempt to acquire a desired system at sequence counts 1,3, 8 and 15. In this example, the 1^(st) specified sequence count is 1,the 2^(nd) specified sequence count is 3, the 3^(rd) specified sequencecount is 8, and the 4^(th) specified sequence count is 15. Such anexemplary system acquisition schedule can be represented, for example,by the function command or script SCHED(PWR.UP, SYS,ACQ_(—)1_(—)3_(—)8_(—)15).

[0073] If, at step 510, the reference counter value is equal to thei^(th) specified sequence count defined by the system acquisitionschedule (that is, if the answer to step 510 is YES), then flow goes toa step 512. At step 512, the subscriber station attempts to acquire thedesired system. As discussed above, the desired system can be, forexample, the last system that was used prior to the triggering event(for example, if the triggering event is a power up), or the system thatwas providing service when the triggering event occurred (for example,if the triggering event was loss of a system). One of ordinary skill inthe art will appreciate that other desired systems can be selected whilebeing within the spirit and scope of the present invention.

[0074] At a next step 514, there is a determination as to whetheracquisition was successful, and whether an affirmative decision was madeto provide service on the acquired system. More specifically, when asystem is successfully acquired, the subscriber station can detect theSID of the acquired system (for example, using an overhead channel). Inone embodiment, the decision is made to use the acquired system only ifthe acquired system is the desired system. In another embodiment, thedecision is made to use the acquired system if the acquired system isassociated with an acceptable or preferred (that is, a non-negative)SID, even if the acquired system is not the “desired system”.

[0075] If the answer to step 514 is YES, service is provided using theacquired system at a step 520. At this point, use of the acquisitionschedule can be terminated, at a step 522.

[0076] If the answer to step 514 is NO, flow returns to step 506 wherepointer value i is incremented. Flow then goes to step 508 where thereference count value is incremented, and then to step 510 where thereis another determination as to whether the reference count value isequal to the i^(th) specified sequence count (for example, the 2^(nd)specified sequence count of 3) defined by the system acquisitionschedule. If the answer to step 510 is YES, then flow goes to step 512as discussed above.

[0077] If, at step 510, the reference counter value is not equal to thei^(th) specified sequence count defined by the system acquisitionschedule (that is, if the answer to step 510 is NO), then flow goes to astep 516. At step 516, the subscriber station attempts to acquire asystem other than the desired system (for example, an alternativesystem). The subscriber station may select an alternative system basedon one or more of the tables or lists maintained by the subscriberstation. The precise manner in which the subscriber station selects analternative system that the subscriber station will attempt acquire isnot important to the present invention.

[0078] At a next step 518, there is a determination as to whetheracquisition was successful, and whether an affirmative decision was madeto provide service on the acquired system. More specifically, when asystem is successfully acquired, the subscriber station can detect theSID of the acquired system (for example, using an overhead channel). Anaffirmative decision can then be made to use the acquired system, forexample, if the acquired system is associated with an acceptable orpreferred (that is, a non-negative) SID.

[0079] If the answer to step 518 is YES, then at step 520 service isprovided using the acquired system. At this point, use of theacquisition schedule can be terminated at step 522.

[0080] If the answer to step 518 is NO, flow returns to step 508 wherethe reference count value is incremented to produce an updated referencecount value. Then, at step 510, there is a determination as to whetherthe updated reference count value is equal to the i^(th) specifiedsequence count of the plurality of reference counts defined by thesystem acquisition schedule.

[0081] In the above discussed manner, the subscriber station willattempt to acquire the desired system when the reference count value isequal to the i^(th) specified sequence count defined by the systemacquisition schedule. If the desired system (or possibly, anotheracceptable system) is not acquired, then the pointer value i isincremented to produce an updated i^(th) pointer value, and thereference count value is incremented to produce an updated referencecount value. Acquisition of the desired system is not again attempteduntil the reference count value is again equal to the i^(th) specifiedsequence count value defined by the system acquisition schedule. Atother reference count values (that is, when the reference count value isless than the i^(th) sequence count value), the subscriber stationattempts to acquire alternative systems. Accordingly, the subscriberstation attempts to acquire the desired system in accordance with thereference counts defined by the system acquisition schedule. In theabove discussed embodiment, the reference counter is incremented eachtime a system acquisition attempt is unsuccessful. One or ordinary skillin the art will appreciate that other types of events can cause thereference counter to be updated. The subscriber station attempts toacquire other systems when the system acquisition does not specify thatthe subscriber station should attempt to acquire the desired system.This shall now be described with reference to the sequence line of FIG.6.

[0082] Referring to FIG. 6, the shaded areas of sequence line 600represent sequence counts at which the subscriber station attempts toacquire the desired system. The non-shaded areas represent sequencecounts at which the subscriber station attempts to acquire alternativesystems. The system acquisition schedule defines sequence counts atwhich a subscriber station should attempt to acquire a desired system,wherein the sequence counts are all relative to the triggering event.For example, the exemplary function command or script SCHED(PWR.UP, SYS,ACQ_(—)1_(—)3_(—)8_(—)15) specifies that the subscriber station shouldattempt to acquire a desired system at when the sequence count equals 1,3, 8 and 15, starting from the triggering event of power up. In analternative embodiment, the system acquisition schedule defines sequencecount values between each successive attempts to acquire a desiredsystem. For example, the exemplary function command or scriptSCHED(PWR.UP, SYS, ACQ1_(—)2_(—)5_(—)7) can specify that the subscriberstation should attempt to acquire a desired system at 1 count after thetriggering event, then 2 counts after the previous attempt (which is 3counts after the triggering event), then 5 counts after the previousattempt (which is 8 counts after the triggering event), then 7 countsafter the previous attempt (which is 15 counts after the triggeringevent). It should be appreciated that the effect of both of theseexamples is to attempt to acquire the desired systems at equivalentcounts relative to the triggering event. One of ordinary skill in theart will appreciate that the flow chart of FIG. 5 can be slightlymodified to accommodate this alternative embodiment. More specifically,the reference counter should be initialized to zero after eachunsuccessful attempt to acquire the desired system (that is, after eachtime the answer to step 514 is NO), as shown by the dashed lines in FIG.5.

[0083] A subscriber station can be searching for more than one desiredsystem in accordance with more than one sequence based systemacquisition schedule. Further, a subscriber station can be searching fora first desired system in accordance with a sequence based systemacquisition schedule while the subscriber station is searching for asecond desired system in accordance with a time based system acquisitionschedule. If both time based and sequence based system acquisitionschedules are utilized, each system acquisition schedule should bespecified as either a time based or sequence based system acquisitionschedule.

[0084] The present invention should not be limited to use with theexemplary triggering events that have been discussed above. Any otherevent that can be recognized by the subscriber station can be specifiedas a triggering event and can have an associated system acquisitionschedule. Examples of additional triggering events include: CDMAacquisition fails during the pilot channel acquisition substate; CDMAacquisition fails during the sync channel acquisition substate; AMPSacquisition attempt fails because neither of the two strongest pagingchannels could be acquired; AMPS acquisition fails because neither ofthe two strongest control channels could be acquired.

[0085] A time based system acquisition schedule is not dependent on theprocessing speed of a subscriber station or on the amount of time ittakes to perform subscriber station functions. In contrast, a sequencebased acquisition schedule is likely dependent on, and thus likelyaffected by, the processing speed of a subscriber station and on theamount of time it takes to perform subscriber station functions.

[0086] Features of the present invention can be performed and/orcontrolled by processor 108, which in effect comprises a computersystem. Such a computer system includes, for example, one or moreprocessors that are connected to a communication bus. Althoughtelecommunication-specific hardware can be used to implement the presentinvention, the following description of a general purpose type computersystem is provided for completeness.

[0087] The computer system can also include a main memory, preferably arandom access memory (RAM), and can also include a secondary memoryand/or other memory. The secondary memory can include, for example, ahard disk drive and/or a removable storage drive. The removable storagedrive reads from and/or writes to a removable storage unit in a wellknown manner. The removable storage unit, represents a floppy disk,magnetic tape, optical disk, and the like, which is read by and writtento by the removable storage drive. The removable storage unit includes acomputer usable storage medium having stored therein computer softwareand/or data.

[0088] The secondary memory can include other similar means for allowingcomputer programs or other instructions to be loaded into the computersystem. Such means can include, for example, a removable storage unitand an interface. Examples of such can include a program cartridge andcartridge interface (such as that found in video game devices), aremovable memory chip (such as an EPROM, or PROM) and associated socket,and other removable storage units and interfaces which allow softwareand data to be transferred from the removable storage unit to thecomputer system.

[0089] The computer system can also include a communications interface.The communications interface allows software and data to be transferredbetween the computer system and external devices. Software and datatransferred via the communications interface are in the form of signalsthat can be electronic, electromagnetic, optical or other signalscapable of being received by the communications interface.

[0090] As shown in FIG. 1, processor 108 is in communications withmemory that stores MRU 109, PRL 112 and/or UST 111. Processor 108 isalso in communications with memory 110, which can store the systemacquisition schedules of the present invention. Processor 108, alone orcombination with analog modulation and processing circuitry 104 and/ordigital modulation and processing circuitry 106, detects triggeringevents that trigger the use of the system acquisition schedules of thepresent invention.

[0091] In this document, the terms “computer program medium” and“computer usable medium” are used to generally refer to media such as aremovable storage device, a removable memory chip (such as an EPROM, orPROM) within MMSS 100, and signals. Computer program products are meansfor providing software to the computer system.

[0092] Computer programs (also called computer control logic) are storedin the main memory and/or secondary memory. Computer programs can alsobe received via the communications interface. Such computer programs,when executed, enable the computer system to perform certain features ofthe present invention as discussed herein. For example, features of theflow charts shown in FIGS. 2, 3 and 5 can be implemented in suchcomputer programs. In particular, the computer programs, when executed,enable processor 108 to perform and/or cause the performance of featuresof the present invention. Accordingly, such computer programs representcontrollers of the computer system of the subscriber station. Thus, suchcomputer programs control, for example, the order in which systemacquisition is attempted.

[0093] Where the invention is implemented using software, the softwarecan be stored in a computer program product and loaded into the computersystem using the removable storage drive, the memory chips or thecommunications interface. The control logic (software), when executed byprocessor 108, causes control processor 108 to perform certain functionsof the invention as described herein.

[0094] Features of the invention may also or alternatively beimplemented primarily in hardware using, for example, hardwarecomponents such as application specific integrated circuits (ASICs).Implementation of the hardware state machine so as to perform thefunctions described herein will be apparent to persons skilled in therelevant art(s).

[0095] The previous description of the preferred embodiments is providedto enable any person skilled in the art to make or use the presentinvention. While the invention has been particularly shown and describedwith reference to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention.

[0096] The present invention has been described above with the aid offunctional building blocks illustrating the performance of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed. Any such alternate boundaries are thus within the scope andspirit of the claimed invention. One skilled in the art will recognizethat these functional building blocks can be implemented by discretecomponents, application specific integrated circuits, processorsexecuting appropriate software and the like or any combination thereof.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

What is claimed is:
 1. For use with a subscriber station (100), a method for attempting to acquire one of a plurality of systems, comprising the steps of: (a) detecting an event that triggers use of a system acquisition schedule, (b) selecting the system acquisition schedule from a plurality of system acquisition schedules based on the triggering event, wherein the system acquisition schedule defines when to attempt to acquire a desired system; (c) attempting to acquire the desired system in accordance with the system acquisition schedule; and (d) performing other functions when the system acquisition schedule does not indicate to attempt to acquire the desired system.
 2. The method of claim 1, further comprising the step of selecting the desired system based on the triggering event.
 3. The method of claim 1, further comprising the steps of: (e) detecting another event that triggers use of a second system acquisition schedule, wherein the second system acquisition schedule defines when to attempt to acquire a second desired system; (f) attempting to acquire the second desired system in accordance with the second system acquisition schedule
 4. The method of claim 3, wherein step (d) comprises performing other functions when the system acquisition schedule does not indicate an attempt should be made to acquire the desired system and the second system acquisition schedule does not indicate an attempt should be made to acquire the second desired system.
 5. The method of claim 1, wherein step (d) comprises attempting to acquire other systems when the system acquisition schedule does not indicate an attempt should be made to acquire the desired system.
 6. The method of claim 5, wherein each system is defined by a mode/frequency combination, the desired system being defined by a first mode/frequency combination, and each of the other systems being defined by different mode/frequency combinations than the desired system.
 7. The method of claim 1, wherein step (a) comprises detecting a system loss event that triggers use of the system acquisition schedule, and wherein the system acquisition schedule defines when to attempt to re-acquire the desired system whose loss triggers use of the system acquisition schedule.
 8. The method of claim 1, wherein step (a) comprises detecting a power up event, and wherein the system acquisition schedule defines when to attempt to acquire the desired system, the desired system being the system that was most recently used by the subscriber station (100).
 9. The method of claim 1, wherein the system acquisition schedule defines specified times at which the subscriber station (100) should attempt to acquire the desired system, and wherein: step (c) comprises attempting to acquire the desired system at the specified times defined by the system acquisition schedule; and step (d) comprises performing other functions at times other than the specified times defined by the system acquisition schedule.
 10. The method of claim 1, wherein the system acquisition schedule defines specific sequence counts according to which the subscriber station (100) should attempt to acquire the desired system, and wherein: step (c) comprises attempting to acquire the desired system according to the specific sequence counts defined by the system acquisition schedule; and step (d) comprises performing other functions at sequence counts other than the specific sequence counts defined by the system acquisition schedule.
 11. The method of claim 1, wherein the system acquisition schedule defines specified times at which the subscriber station (100) should attempt to acquire the desired system, and wherein: step (c) comprises attempting to acquire the desired system at the specified times defined by the system acquisition schedule; step (d) comprises attempting to acquire the other systems at times other than the specified times defined by the system acquisition schedule; and terminating steps (c) and (d) once the desired system, or one of the other systems, is acquired.
 12. The method of claim 1, wherein the system acquisition schedule defines specific sequence counts according to which the subscriber station (100) should attempt to acquire the desired system, and wherein: step (c) comprises attempting to acquire the desired system according to the specific sequence counts defined by the system acquisition schedule; step (d) comprises attempting to acquire the other systems at sequence counts other than the specific sequence counts defined by the system acquisition schedule; and terminating steps (c) and (d) once the desired system, or one of the other systems, is acquired.
 13. The method of claim 12, further comprising the steps of: (e) detecting another event that triggers use of a second system acquisition schedule, wherein the second system acquisition schedule defines when the subscriber station should attempt to acquire a second desired system, the second desired system defined by a mode/frequency combination that is different than a mode/frequency combination defining the desired system; (f) attempting to acquire the second desired system in accordance with the second system acquisition schedule; and (g) attempting to acquire the other systems when the second system acquisition schedule does not indicate the subscriber station should attempt to acquire the second desired system.
 14. A computer program product comprising a computer usable medium having control logic stored therein for causing a computer of a subscriber station (100) to attempt to acquire one of a plurality of systems, the control logic comprising: first computer readable program code means for causing the computer to use a system acquisition schedule when a triggering event is detected; second computer readable program code means for causing the computer to select the system acquisition schedule from a plurality of system acquisition schedules based on the triggering event, wherein the system acquisition schedule defines when to attempt to acquire a desired system; third computer readable program code means for causing the computer to attempt to acquire the desired system in accordance with the system acquisition schedule; and fourth computer readable program code means for causing the computer to perform other functions when the system acquisition schedule does not indicate an attempt should be made to acquire the desired system.
 15. The computer program product of claim 14, further comprising: fifth computer readable program code means for causing the computer to select the desired system based on the triggering event.
 16. The computer program product of claim 14, wherein the system acquisition schedule defines when the third computer readable program code means causes the computer to attempt to re-acquire the desired system whose loss comprises the triggering event.
 17. An apparatus for use in a subscriber station (100) attempting to acquire one of a plurality of systems, the subscriber station (100) capable of wireless communications, the improvement comprising: detecting means for detecting an event that triggers use of a system acquisition schedule, wherein the system acquisition schedule defines when to attempt to acquire a desired system, the desired system being defined by a mode/frequency combination; selecting means for selecting the system acquisition schedule from a plurality of system acquisition schedules based on the triggering event, wherein the system acquisition schedule defines when to attempt to acquire a desired system; and acquisition means for attempting to acquire the desired system in accordance with the system acquisition schedule, wherein the subscriber station (100) performs other function when the acquisition schedule does not indicate an attempt should be made to acquire the desired system.
 18. The apparatus of claim 17, wherein the acquisition means attempts to acquire other systems when the acquisition schedule does not indicate an attempt should be made to acquire the desired system.
 19. The apparatus of claim 18, wherein the system acquisition schedule is a time based system acquisition schedule that defines times at which the acquisition means is to attempt to acquire the desired system.
 20. The apparatus of claim 18, wherein the system acquisition schedule is a sequence based system acquisition schedule that defines when to attempt to acquire the desired system.
 21. For use with a subscriber station (100), a method for attempting to acquire one of a plurality of systems, comprising the steps of: (a) detecting an event that triggers use of a system acquisition schedule, wherein the system acquisition schedule defines specific times at which the subscriber station (100) should attempt to acquire a desired system; (b) attempting to acquire the desired system at the specific times defined by the system acquisition schedule; and (c) attempting to acquire one or more other systems at times other than the specific times defined by the system acquisition schedule.
 22. The method of claim 21, wherein the desired system is defined by a mode/frequency combination, and each of the one or more other systems is defined by a different mode/frequency combination.
 23. The method claim 21, further comprising stopping steps (b) and (c) once the system, or one of the one or more other systems, is acquired.
 24. The method of claim 21, wherein the desired system is a system whose loss triggers use of the system acquisition schedule.
 25. The method of claim 21, wherein the event that triggers use of the system acquisition schedule comprises a power-up of the subscriber station (100), and wherein the desired system is a system that was most recently used by the subscriber station (100).
 26. For use with a subscriber station (100), a method for attempting to acquire one of a plurality of systems, comprising the steps of: (a) detecting an event that triggers use of a system acquisition schedule, wherein the system acquisition schedule defines specific sequence steps at which the subscriber station (100) should attempt to acquire a desired system; (b) attempting to acquire the desired system at the specific sequence steps defined by the system acquisition schedule; and (c) attempting to acquire the one or more other systems at sequence steps other than the specific sequence steps defined by the system acquisition schedule.
 27. The method of claim 26, wherein the desired system is defined by a mode/frequency combination, and each of the one or more other systems is defined by a different mode/frequency combination.
 28. The method of claim 26, further comprising stopping steps (b) and (c) once the desired system, or one of the one or more other systems, is acquired.
 29. The method of claim 26, wherein the desired system is a system whose loss triggers use of the system acquisition schedule.
 30. The method of claim 26, wherein the event that triggers use of the system acquisition schedule comprises a power-up of the subscriber station (100), and wherein the desired system is a system that was most recently used by the subscriber station (100). 